1. Field of the Invention
This invention relates to an image display apparatus comprising a light source and a transmission type optical modulation panel and a method of driving the same.
2. Related Background Art
Image display apparatuses comprising a transmission type optical modulation panel such as a liquid crystal display panel to be illuminated from behind to show images are known. Such display apparatuses are normally classified into a direct viewing type comprising an image carrying panel to be directly viewed and a projection type comprising a screen for showing projected images.
As a matter of course, such image displays are required to show satisfactorily bright images without uneven distribution of illumination.
FIG. 37 of the accompanying drawings schematically shows a conventional direct viewing type image display apparatus. Referring to FIG. 37, the apparatus comprises a fluorescent lamp 641, a reflector 642 having a concave cross section, a diffusing panel 643 and a liquid crystal display panel 644. Rays of light emitted from the fluorescent lamp 641 are partly made to hit the diffusing panel 643 directly and partly reflected by the reflector 642 before they get to the diffusing panel 643. The liquid crystal display panel 644 is illuminated from behind by the rays of light that have been diffused by the diffusing panel 643.
However, it is a known fact that it is practically impossible for an image display apparatus having a configuration as described above to show satisfactorily bright images without uneven distribution of illumination, even if the fluorescent lamp 641 and the reflector 642 are designed optimally in terms of brightness and distribution of illumination. A net result is then an unevenly illuminated display panel.
On the other hand, it is desired for a projection type image display apparatus to have its transmission type optical modulation panel illuminated brighter than that of a direct viewing type apparatus with an enhanced level of even distribution of illumination. According to a known method developed to achieve this objective, a highly bright luminous tube such as a metal halide lamp and a concave mirror are combined to produce a light source for illuminating a liquid crystal display panel. With such an arrangement, however, the metal halide lamp itself intercepts some of the rays of light reflected by the concave mirror to project its shadow on the panel and hence hinders the attempt of evenly illuminating the panel.
Another known illumination method involves the use of a cathode luminescence light source. FIG. 38 of the accompanying drawings schematically shows a display apparatus using such a light source. Referring to FIG. 38, the apparatus comprises a glass vacuum container 701, a phosphor layer 702, a metal backing layer 703 and an electron gun having a thermionic cathode as an electron source. Again, however, it is practically impossible for such a light source to irradiate the phosphor layer with evenly distributed electron beams, even if its electronic optical lens and electron gun are designed optimally in terms of brightness and distribution of illumination. A net result is also an uneven illumination.
There has also been proposed a cathode luminescence light source for a display apparatus as schematically illustrated in FIG. 39 of the accompanying drawings in an attempt to achieve an evenly distributed illumination. In FIG. 39, reference numerals 701 through 703 denote, respectively, the same components as those of FIG. 38, although a plurality of wire cathodes (linear thermionic cathodes) 805 are arranged in parallel to replace the electron gun 704 of FIG. 38. Additionally, a grid electrode 806 is provided to improve the even distribution of electron beams irradiating the phosphor layer 702. However, the display panel of this apparatus is accompanied by regularly distributed variations in the intensity of illumination attributable to the arrangement of wire cathodes 805 and variations in the temperature of the wire cathodes 805 (mainly attributable to the heat loss that takes place at the wire cathode supporting sections) and it is extremely difficult, if not totally impossible, to completely eliminate these variations. Since each of the light valves of a liquid crystal display panel is very small in its effect when compared with the wire cathodes 805, the regularly distributed variations in the intensity of illumination of the panel are directly reflected on the brightness of the images projected on the panel.